1. Field of the Invention
The present invention relates to glutamic acid decarboxylase.sub.65 (GAD.sub.65) polypeptides and methods of using GAD.sub.65 polypeptides diagnostically and therapeutically in autoimmune disease.
2. Description of the Background Art
Insulin-dependent diabetes mellitus (IDDM; type I diabetes) is one of the most common metabolic disorders. In the United States, IDDM affects approximately one in 300 to 400 people, and epidemiological studies suggest that its incidence is increasing. The disease results from the autoimmune destruction of the insulin-producing .beta.-cells of the pancreas. More specifically, the preonset stage is characterized by "insulitis", in which lymphocytes infiltrate the pancreatic islets and selectively destroy the .beta.-cells. Insulitis may be present for many years before the onset of clinical symptoms. The typical IDDM presentation of hyperglycemia appears only after at least 80% of the insulin-producing .beta.-cells are lost. The remaining .beta.-cells are destroyed during the next few years.
Although insulin therapy allows most IDDM patients to lead normal lives, this replacement is imperfect and does not completely restore metabolic homeostasis. Thus, severe complications which result in dysfunctions of the eye, kidney, heart, and other organs are common in IDDM patients undergoing insulin therapy. Because of this, it is highly desirable to extend the latency period and prevent progression (e.g., through administration of immunosuppressant drugs to interfere with the autoimmune process and insulin to achieve better control of the effects of sustained hypoglycemia) between the start of .beta.-cell destruction and the actual requirement of insulin replacement (i.e., when 80% of the .beta.-cells are destroyed). Therefore, a diagnostic test which determines the beginning of .beta.-cell destruction would allow the clinician to administer immunosuppressant drugs (Silverstein, et al., New England Joumal of Medicine, 319:599-604, 1988) or prophylactic insulin therapy (Keller, et al., Lancet, 341:927, 1993) to extend this latency period and thus significantly delay the onset of insulin replacement side effects.
Many IDDM patients have sera which contain antibodies to a 64 kD molecule (Baekkeskov, et al., J.Clin.Invest., 79:926-934, 1987; Atkinson, et al., Lancet, 335:1357-1360, 1990), to islet cell cytoplasmic (ICA) molecules or islet cell surface (ICSA) molecules (Bottazzo, et al, Lancet, 1:668-672, 1980), or to insulin (Palmer, et al., Science, 222:1137-1139, 1983; Atkinson, et al., Diabetes, 35:894-898, 1986). Atkinson and coworkers (Atkinson, et al., Lancet, 335:1357-1360, 1990) have demonstrated that the presence of antibodies to the 64 kD molecule in human sera appears to be the earliest and most reliable indicator that onset of IDDM symptoms will eventually occur.
Recently, Baekkeskov and coworkers established that the 64 kD molecule and glutamic acid decarboxylase (GAD) have several antigenic epitopes in common and thus they may be identical or very similar molecules. Although this identification is an important finding, the use of this information as a diagnostic tool for predicting IDDM is quite cumbersome and limited unless knowledge of the molecular biology of GAD is known. Studies by Kaufman, et al., (J. Clin. Invest., 8:283, 1992) established that the 64 kD molecule was intact GAD.sub.65. Consequently, the cloning and subsequent production of large quantities of GAD.sub.65 or a GAD molecule which is antigenically substantially identical to the GAD.sub.65 molecule or fragments of the GAD.sub.65 molecule, both of which can be easily purified, will allow the development of a diagnostic kit designed to predict IDDM as well as effective therapeutic modalities. The present invention provides a means for accomplishing these results.